This invention pertains generally to semiactive guidance control systems for guided missiles and particularly to a system of such kind in which the operating frequency of a reference oscillator in a guided missile during flight is controlled to maintain coherence between such operating frequency and the frequency of a control radar.
It is known in the art, as shown in the now pending U.S. patent application, Ser. No. 579,281, entitled "Adaptive Semiactive Missile Guidance System and Elements Therefore", and assigned to the same assignee as this application, that an electronically tunable arrangement may be used in a guided missile to maintain coherence between the operating frequency of a reference oscillator in such missile during flight and the frequency of a control radar. As described in detail in that application, the electronically tunable arrangement includes anYttrium-Iron-Garnet (or YIG) filter as the frequency determining element for the reference oscillator. Briefly, in such an arrangement, portions of the output signal from a voltage-controlled oscillator (the first local oscillator) are passed through a YIG filter and a passive phase shifter to the input terminals of a phase detector. The signal out of such detector, then, is indicative of the difference between the frequency of the output signal of the voltage controlled oscillator (the first local oscillator signal) and the resonant frequency of the YIG filter. The signal out of the phase detector then, after appropriate shaping, is applied to the voltage controlled oscillator to force the frequency of the output signal from that element into coincidence with the resonant frequency of the YIG filter.
In the system contemplated in the referenced patent, tuning of the voltage controlled oscillator to the proper frequency with respect to the frequency of the radar signal from the control radar is effected after the guided missile is launched by changing the resonant frequency of the YIG filter in a programmed manner until an output signal is produced by the rear receiver. When, as in the system contemplated in the referenced patent, the control radar is either a continuous wave or a pulse radar, the requisite changes in the resonant frequency of the YIG filter may be effected by appropriately controlling the current through a tuning coil in such a filter. That is to say, when the frequency of the radar signal of a control radar is substantially constant, the strength of the magnetic field in a YIG filter may be satisfactorily adjusted by controlling the current through the tuning coil in such a filter.
A different situation may obtain, however, when coding of the radar signal of the control radar is changed. For example, when the control radar is mounted on a ship, it may be desirable to transmit closely spaced frequency modulated pulses, commonly called "chirp" pulses. With such coding it is necessary to change the resonant frequency of the YIG filter by a substantial amount, say in the order of 2 MHz. Obviously, if either the pulse length or the interval between successive pulses is, relatively, very short, the current through the tuning coil must be changed relatively rapidly. Unfortunately, in such a situation the inductance of the tuning coil and unavoidable losses in the core of any known YIG filter militates against the attainment of any relatively rapid change in current with a current source of reasonable size.
It is theoretically possible to redesign any known YIG filter to allow the resonant frequency of such a device to be changed more quickly. Thus, if an ancillary tuning coil with an air core were to be added, the inductance and core losses of such coil would be far less than the inductance and core losses of the primary tuning coil in the filter. Practical considerations, however, rule out the addition of an ancillary tuning coil. In particular, if an ancillary tuning coil were to be added, the resulting filter would be extremely expensive, would be of questionable reliability and would be susceptible to microphonics.